Conventional seismic technology uses surface sources of seismic energy and surface receivers to detect reflections from underground geologic formations such as layers and faults. The collected seismic data can be used to locate the underground formations and guide drilling operations to sources of hydrocarbons.
In Vertical Seismic Profiling ("VSP"), drilling operations are interrupted to place a series of seismic sensors at discrete depths in a borehole being drilled. A surface source releases energy that is reflected off underground geologic formations. The seismic sensors in the borehole sense the reflected energy and provide signals representing the reflections to the surface for analysis. The seismic data is used in analysis of the geology of the earth near the borehole.
In a subsequent development, known as "drill bit seismics", seismic sensors were positioned at the surface near the borehole to sense seismic energy imparted to the earth by the drill bit during drilling. The sensed energy was used in the traditional seismic way to detect reflections from underground geologic formations. Further, this technique was used to detect "shadows", or reduced seismic energy magnitude, caused by underground formations, such as gas reservoirs, between the drill bit and the surface sensors.
Analysis of drill bit seismic data entails determining the amount of time between the generation of the seismic energy by the drill bit and the detection of the seismic energy. This can be accomplished by using a "pilot sensor" near the top of the drillstring to which the drill bit is attached. The pilot sensor detects the vibrations transmitted through the drillstring by the drill bit. Those vibrations, corrected and filtered to account for the delay and distortion caused by the drillstring, can be correlated with the seismic energy received at the surface sensor to determine the distance traveled by the seismic energy received at the surface sensors. With this information, the location of the underground formations can be determined.
Drill bit seismic techniques were originally applied to vertical and near-vertical wells where the drillstring is generally not in contact with the borehole. Contact between the drillstring and the borehole wall attenuates the drill bit signal received by the pilot sensor, modifies the drillstring transfer function, and causes spurious signals to be transmitted into the formation. Hence, drill bit seismics have not been effective under those conditions.